# Priority rules

Priority rules are used to assign priority numbers according to certain sequence criteria, according to which the jobs in the queue in front of a machine are then processed. The most important priority rules are:
1. FCFS rule: With the "First-Come-First-Served" rule, the order that arrives first is assigned the highest priority. The orders are processed in the order in which they arrive at the respective machine.

2. GRB rule: With the “largest remaining processing time” rule, the order is assigned the highest priority number that has the longest remaining processing time on all required machines at the time of booking.

3. KRB rule: The "Shortest remaining processing time" rule assigns the highest priority number to the order, the processing time of which is the shortest on all machines required at the time of occupancy.

4. MAA rule: This priority rule gives the job in the queue the highest priority number. which includes most of the operations still to be carried out.

5. WAA rule: This priority rule assigns the highest priority number to the job in the queue, which contains the fewest operations to be carried out.

6. LOZ rule: With the "Longest operation time" rule, the order that has the longest processing time on the machine in question receives the highest priority.

7. KOZ rule: The "shortest operation time" rule assigns the highest priority to the order with the shortest operation time.

B. GGB rule: The job in the queue that has the greatest total processing time on all machines receives the highest priority number.

9. KGB rule: The order with the shortest total processing time on all machines receives the highest priority number.

10. FFT rule: The order with the earliest completion date is assigned the highest priority number.

11. SZ rule: The order in the queue receives the highest priority where the difference between the delivery date and the remaining processing time, i.e. its slack, is the smallest.

12. Value rule: The highest priority number is given to the order that either has the highest final product value or the product value of which is the highest before the respective operation is carried out (dynamic value rule).

Simple priority rules are mostly aimed at a relatively one-sided pursuit of certain goals of sequence planning. For this reason, a combination of elementary work priority rules is often made. The simplest form of the link is the addition of two elementary priority numbers. However, since the value ranges of different priority rules are usually different, the two components must be weighted. This is done by multiplying by a constant or by an exponential factor. Another form of linking is the multiplication of priority numbers. In this case, weighting can only be carried out using different exponents.

In simulations it could be shown that the undesired effects of individual rules in the case of the additive and multiplicative linkage are partially increased. As a rule, therefore, only the alternative link is suitable as a form of combination, in which the more favorable rule with regard to the pursued goal comes into effect. The shortest operation time rule, the dynamic value rule and the slack time rule have proven to be particularly recommendable with regard to individual optimization goals.

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